Photonic crystal fibers, herein referred to as PCF, belong to a class of fibers comprising optical nano or micro structures that affect the motion photons. PCF (sometimes called holey fiber, hole-assisted fiber, microstructure fiber, or microstructured fiber) at least partly obtains its waveguide properties by an arrangement of microstructures e.g. in the form of air (or gas) holes or solid microstructures with a refractive index differing from the surrounding background material. There is a great variety of hole/microstructure arrangements, leading to PCFs with very different properties.
Examples of PCFs include the PCFs described in U.S. Pat. No. 6,985,661, 8,938,146 or 7,792,408.
The termination of such PCFs is often rather difficult in particular due to the microstructures and optionally hollow microstructures and/or a hollow core.
Terminal structures for traditional step-index fibers, i.e. optical fibers with a core having a uniform refractive index and a surrounding cladding having a lower refractive index providing a sharp decrease in refractive index at the core-cladding interface, are well known. U.S. Pat. No. 4,737,011 discloses a connector for a large core step-index fiber designed such that high power light pumped into the end of the fiber has low risk in burning or melting the connector material. The connector comprises a holder with a metallic plug body and a radially inwardly arranged sleeve having a support portion adapted to support the optical fiber at a distance from the end-facet of the optical fiber and wherein the support portion employs a transparent or a translucent heat resistant inorganic substance e.g. ceramic material or sapphire having a melting point of 1500° C. or more and having a refractive index higher than that of the cladding of the optical fiber. It is not described how the fiber is mounted in the connector.
Generally it is desired to terminate an optical fiber such that it is simple to handle and is sufficiently protected against dust, moisture and heat.
The small diameter and core diameter of the PCF and its typically high flexibility require that a termination of the PCF is held in a mechanically rigid structure—normally called a ferrule or a ferrule structure—at termination points in order to be practically useful in precise beam delivery systems.
U.S. Pat. No. 7,242,835 discloses a fiber termination combination which includes an optical fiber having a fiber core for transmitting a highly energetic optical signal that can damage the fiber, and a structured region around the core for directing the optical signal into the core, the structured region being characterized by multiple channels of smaller internal diameter than the core defined by thin walls disposed around the core; a ferrule, with an opening therein for locating the fiber, at the end of the fiber enveloping the fiber extremity which cooperates with the blocking structure to block the optical signal from impinging on the microstructured region of the fiber; and a blocking structure disposed over the end of the fiber with an opening mating with the fiber core, the blocking structure blocking the optical signal from impinging on the microstructured region of the fiber.
U.S. Pat. No. 7,373,062 discloses an optical fiber which comprises a hollow fiber core, wherein the front faces of both fiber ends of the hollow fiber core are open and each fiber end is surrounded by a protection element in a dustproof fashion. The protection element includes a window at its front face in front of the fiber end to couple and decouple light to and from the hollow fiber core.
U.S. Pat. No. 7,306,376 discloses a monolithic optical ferrule wherein a fiber is terminated bonded by fusion to form a monolithic unit which minimizes optical loss and is typically capable of transmitting high power laser radiation, preferably in the order of 500 W and higher, without damage to the fiber and ferrule. The end cap, fiber and fusible powder are composed of material of substantially the same physical characteristics such that, when all are fused together, the structure so formed is monolithic and the optical path is transparent.
The prior art fiber terminations disclosed above are generally difficult to mount to the fiber and often result in damaging of the fiber or result in a poor alignment in the z-direction (the axial direction of the fiber) and/or a poor anchoring of the optical fiber which result in a poor coupling of the fiber to another element.